This research will study the enzymatic repair of damage to the DNA of the vegatative and developing cells of the cellular slime mold, Dictyostelium discoideum, after treatment with the mutagenic and carcinogenic agents, ultraviolet light, gamma rays, methyl methane-sulfonate, N-Methyl-N-Nitrosourea, 4-Nitroquinoline-1-oxide, and N-Acetoxy-2-Acetylamino-fluorene. Repair in the non-replicating developing cells at various stages will be compared to that in the replicating vegetative cells. Repair will be measured by excision of altered bases, appearance and sealing of single strand breaks, unscheduled DNA synthesis during development, and repair replication using DNA density-shift techniques. Repair-capable and repair defective strains will be compared. A correlation will be sought between unrepairable damage and the appearance of mutations in the haploid cells following treatment during development. Forward mutations from methanol-sensititivity to methanol resistance (acr A locus) will be quantitated following treatment with the physical and chemical agents listed above. Developmental mutants and reversions to resistance from temperature-sensitive and radiation-sensitive strains will also be investigated. To facilitate more efficient specific labeling of the DNA of D. discoideum, thymidine and thymidine monophosphate metabolism and pools will be studied with and without various inhibitors. Auxotrophic mutants requiring these precursors will be sought.